In recent years, mobile communication terminal equipment, such as a portable telephone, and the like has been rapidly progressed. From the viewpoint of convenience of portability or handiness, it is strongly desired that the mobile communication terminal equipment is not only small in size but also light in weight. In order to fabricate such small and light terminal equipment, it is essentially required that an electronic part or device used therein is also small in size and light in weight. Under the circumstances, many SAW devices each of which has advantage for being fabricated small and light, namely, many SAW filters have been used in a high-frequency section and an intermediate-frequency section of the terminal equipment.
An insertion loss and attenuation outside the pass band can be pointed out as important characteristics required for the SAW filter. Herein, the insertion loss influences power consumption of the mobile communication terminal equipment including the SAW filter. The insertion loss becomes lower, the life of a battery included in the mobile communication terminal equipment becomes longer. Therefore, a capacity of the battery can be reduced, dependent on the insertion loss. Accordingly, the mobile communication terminal equipment can be fabricated small in size and light in weight, when the insertion loss can be low. Further, when wide range attenuation outside the pass band can be obtained in an SAW filter, the mobile communication terminal equipment can also be fabricated small in size and light in weight.
As an SAW filter according to a first prior art, which complies with the characteristics of low insertion loss and high attenuation, there is such a RADA-type filter as disclosed in, for example, Japanese laid open Official Gazette No. Hei 5-183380. The RADA-type filter is such a circuit of multiple stages each composed of a SAW resonator connected to input/output electrodes in serial and another SAW resonator connected to the input/output electrodes in parallel. Plenty of the RADA-type filters have been used in a high-frequency filter of a portable telephone, since the RADA-type filters has excellent characteristics of low insertion loss and attenuation near the pass band thereof.
As an SAW filter according to a first prior art, which complies with the characteristics of low insertion loss and high attenuation, there is such a ladder-type filter as disclosed in, for example, Japanese laid open Official Gazette No. Hei 5-183380. The ladder-type filter is such a circuit of multiple stages each composed of a SAW resonator connected to input/output electrodes in serial and another SAW resonator connected to the input/output electrodes in parallel. Plenty of the ladder-type filters have been used in a high-frequency filter of a portable telephone, since the ladder-type filters has excellent characteristics of low insertion loss and attenuation near the pass band thereof.
The ladder-type filter has an excellent characteristic of the attenuation near the pass band. However, the ladder-type filter has a problem that an attenuation characteristic in a higher frequency region is rapidly deteriorated. Herein, attenuation can be obtained by using LC serial resonance caused by electrostatic capacity C of SAW resonators connected to each other in parallel and inductor component L included in a wiring from a ground electrode on an element to a reference voltage of a package. However, the attenuation characteristics are drastically deteriorated in a case that characteristics are deteriorated due to some reason by serial resistance component included in the wiring. It is therefore difficult to obtain stable attenuation characteristics.
Further, the notch-type filter is advantageous in attenuation characteristics at the higher frequency side of the pass band. However, the characteristics are attenuating smoothly at the lower frequency side of the pass band. The notch-type filter has a problem that band pass characteristics cannot be readily obtained. Further, in the notch-type filter, frequency of attenuation band is limited to anti-resonant frequency of a resonator. The notch-type filter cannot provide so much freedom of design.
Accordingly, it is an object of the present invention to solve these problems of the first and the second prior arts, and to provide an SAW element, an SAW device and a branching filter each having the characteristics of low insertion loss and high attenuation.